1. Field of the Invention
The present invention relates to integrated cracking systems and processes that combine hydrocracking and fluidized catalytic cracking operations, in particular for enhanced flexibility in the production of light olefinic and middle distillate products.
2. Description of Related Art
Hydrocracking processes are used commercially in a large number of petroleum refineries. They are used to process a variety of feeds boiling in the range of 370° C. to 520° C. in conventional hydrocracking units and boiling at 520° C. and above in the residue hydrocracking units. In general, hydrocracking processes split the molecules of the feed into smaller, i.e., lighter, molecules having higher average volatility and economic value. Additionally, hydrocracking processes typically improve the quality of the hydrocarbon feedstock by increasing the hydrogen to carbon ratio and by removing organosulfur and organonitrogen compounds. The significant economic benefit derived from hydrocracking operations has resulted in substantial development of process improvements and more active catalysts.
Mild hydrocracking or single stage once-through hydrocracking occurs at operating conditions that are more severe than hydrotreating processes, and less severe than conventional full pressure hydrocracking processes. This hydrocracking process is more cost effective, but typically results in lower product yields and quality. The mild hydrocracking process produces less mid-distillate products of a relatively lower quality as compared to conventional hydrocracking. Single or multiple catalysts systems can be used depending upon the feedstock processed and product specifications. Single stage hydrocracking is the simplest of the various configurations, and is typically designed to maximize mid-distillate yield over a single or dual catalyst systems. Dual catalyst systems can be deployed as a stacked-bed configuration or in multiple reactors.
In a series-flow configuration the entire hydrotreated/hydrocracked product stream from the first reactor, including light gases (e.g., C1-C4, H2S, NH3) and all remaining hydrocarbons, are sent to the second reactor. In two-stage configurations the feedstock is refined by passing it over a hydrotreating catalyst bed in the first reactor. The effluents are passed to a fractionator column to separate the light gases, naphtha and diesel products boiling in the temperature range of 36° C. to 370° C. The hydrocarbons boiling above 370° C. are then passed to the second reactor for additional cracking.
In fluidized catalytic cracking (FCC) processes, petroleum derived hydrocarbons are catalytically cracked with an acidic catalyst maintained in a fluidized state, which is regenerated on a continuous basis. The main product from such processes has generally been gasoline. Other products are also produced in smaller quantities via FCC processes such as liquid petroleum gas and cracked gas oil. Coke deposited on the catalyst is burned off at high temperatures and in the presence of air prior to recycling regenerated catalyst back to the reaction zone.
In recent years there has been a tendency to produce, in addition to gasoline, light olefins by FCC operations, which are valuable raw materials for various chemical processes. These operations have significant economic advantages, particularly with respect to oil refineries that are highly integrated with petrochemical production facilities.
There are different methods to produce light olefins by FCC operations. Certain FCC operations are based on a short contact time of the feedstock with the catalyst, e.g., as disclosed in U.S. Pat. Nos. 4,419,221, 3,074,878, and 5,462,652, which are incorporated by reference herein. However, the short contact time between feedstock and catalyst typically results in relatively low feed conversion.
Other FCC operations are based on using pentasil-type zeolite, for instance, as disclosed in U.S. Pat. No. 5,326,465, which is incorporated by reference herein. However, the use of a pentasil-type zeolite catalyst will only enhance the yield of light fraction hydrocarbons by excessive cracking of the gasoline fraction, which is also a high value product.
Additional FCC operations are based on carrying out the cracking reactions at high temperature, such as that disclosed in U.S. Pat. No. 4,980,053, which is incorporated by reference herein. However, this method can result in relatively high levels of dry gases production.
Further FCC operations are based on cracking the feed oil at high temperature and short contact time and using a catalyst mixture of specific base cracking catalyst and an additive containing a shape-selective zeolite, as disclosed in U.S. Pat. No. 6,656,346, which is incorporated by reference herein. Processes based on this method are also known as High Severity Fluidized Catalytic Cracking (HS-FCC). Features of this process include a downflow reactor, high reaction temperature, short contact time, and high catalyst to oil ratio.
Downflow reactors permits higher catalyst to oil ratio, since lifting of solid catalyst particles by vaporized feed is not required, and this is particularly suitable for HS-FCC. In addition, HS-FCC processes are operated under considerably higher reaction temperatures (550° C. to 650° C.) as compared to conventional FCC processes. Under these reaction temperatures, two competing cracking reactions occur, thermal cracking and catalytic cracking. Thermal cracking contributes to the formation of lighter products, such as dry gas and coke, whereas catalytic cracking increases propylene and butylene yield. The short residence time in the downflow reactor is also favorable to minimize thermal cracking. Undesirable secondary reactions such as hydrogen-transfer reactions, which consume olefins, are suppressed. The desired short residence time is attained by mixing and dispersing catalyst particles and feed at the reactor inlet followed by immediate separation at the reactor outlet. In order to compensate for the decrease in conversion due to the short contact time, the HS-FCC process is operated at relatively high catalysts to oil ratios.
While individual and discrete hydrocracking and FCC processes are well-developed and suitable for their intended purposes, there nonetheless remains a need for increased flexibility, efficiency and high-value product yield in refinery operations.